Hello,
There are 2 product attributes (combo box). First product attribute contain Car Model. When visitor select the car model "BMW", the 2nd product attribute will only contain "1 series", "3 series", "5 series", "6 series", "7 series".
Sorry for poor english speaking and explaination. Hope this link (http://www.wiperblades.co.uk/) will help you understand my question.